1. Field of the Invention
This invention relates to the optical construction of a plural-beam optical head in an optical recording-reproducing apparatus, and particularly to an optical head for dividing a light beam emitted from a light source into a plurality of beams.
2. Related Background Art
Recently, a parallel writing/reading optical head, in which a plurality of light beams are applied in the form of a spot to a recording medium and the recording and reproduction of information are effected on a plurality of information tracks at a time to thereby improve the transfer rate of the information, has been developed as the optical head of an optical recording-reproducing apparatus. The greater the number of light beams, the greater becomes the number of information tracks on which reading or writing is effected at a time and thus, the transfer rate of information is improved. As methods of generating a plurality of light beams, there are broadly two methods, i.e., a method using a light source having a plurality of light emitting sources or a plurality of light sources, and a method of dividing a light beam from a single light emitting source into a plurality of light beams. As an example of the former, there is a method using a combination of a plurality of LDs (semiconductor lasers) and optical elements such as prisms, or a method using an LD array having a plurality of light emitting points. Of these methods, the method using a combination of a plurality of LDs and prisms or the like suffers from the disadvantage that the number of the optical elements used is increased and not only the construction of the optical head becomes complicated, but also the alignment of the light beams emitted from the individual LDs becomes necessary.
The method using an LD array will hereinafter be described with reference to FIG. 1 of the accompanying drawings. FIGS. 1A and 1B are views in which parts of an optical head optical system according to the prior art are projected onto an xz plane and an xy plane, respectively. The reference numeral 21 designates an LD array having a plurality of light emitting points, the reference numeral 22 denotes a collimator lens, the reference numeral 23 designates a beam splitter serving also as a beam shaping device, the reference numeral 24 denotes an image rotating prism, the reference numeral 25 designates an objective lens, and the reference numeral 26 denotes a recording medium carrying information thereon. FIG. 1C shows a portion of the surface of the recording medium 26. Information tracks 28 are arranged side by side on the recording medium 26.
A divergent light beam emitted from the plurality of light emitting points of the LD array passes through the collimator lens 22, whereby it becomes a plurality of parallel beams conforming to the angle of view with respect to the optical axis of the collimator lens. These parallel beams have their cross-sectional shapes shaped by the beam splitter 23 so as to be substantially circular, whereafter they pass through the image rotating prism 24 and are condensed by the objective lens 25, and form a plurality of spots corresponding to the light emitting points of the LD array on the recording medium 26. The light from these spots is subjected to modulation such as a change in the quantity of reflected light or the rotation of the direction of polarization correspondingly to the information recorded on the information tracks 28, and is made into a parallel beam as a reflected beam via the objective lens 25, whereafter it passes through the image rotating prism 24 and is deflected in the direction of arrow 27 by the beam splitter 23. This deflected light is detected by detecting means, not shown, and the amount of modulation such as the change in the quantity of reflected light or the rotation of the direction of polarization is detected by detecting means, not shown. These detecting means are well known.
When the angle of view of the light emitting points of the LD array with respect to the collimator lens 22 is small, the spacing d between the spots formed on the recording medium 26 is given by the following expression: EQU d.congruent.(f.sub.o /f.sub.c)1/K)L,
where f.sub.o and f.sub.c are the focal lengths of the objective lens 25 and the collimator lens 22, respectively, L is the spacing between the light emitting points on the LD array 21, and K is the shaping ratio for shaping the elliptical cross-section beam emitted from the LD into a circular cross-section, this shaping ratio being of the order of 1.5-2. Due to various conditions for the optical head, particularly compactness, light weight, the utilization efficiency of the light emitted from the LD, etc., the value of (f.sub.c /f.sub.o) suitable at present is of the order of 2-3. Also, the spacing L between the light emitting points of the LD array 21 is limited to the order of 100 .mu.m by thermal crosstalk between chips the adjustment accuracy of chip position, etc. Thus, the limit of the inter-spot spacing d on the recording medium 26 is of the order of 20-30 .mu.m.
On the other hand, it is preferable for the improvement of information recording density that the arrangement pitch P of the information tracks 28 be as small as possible, and for example, in an optical disk, the pitch P is of the order of 1.6 .mu.m. Therefore, in order to effect the parallel reading-out of adjacent information tracks 28, there is adopted an arrangement as shown in FIG. 1C wherein the direction of arrangement of the light beam spots is inclined by an angle 2.theta. with respect to the direction in which the information tracks extend. This inclination of the direction of arrangement of the spots is created by the image rotating prism 24. By the image rotating prism 24 being rotated by an angle .theta. about a direction x, the inclination by 2.theta. is created in the direction of arrangement of the spots on the recording medium 26.
The relation between the information track pitch P and the angle of rotation of the image is EQU P=dsin2.theta. (1)
and when d=30 .mu.m and P=1.6 .mu.m, .theta. is approximately 1.5.degree.. Also, by the differentiation of the above equation (1), the relation between the irregularity of the angle of rotation 0 of the image rotating prism 24 and the irregularity of the pitch P thereby is EQU .DELTA.P=2dcos2.theta..multidot.d.theta.. (2)
Thus, to effect adjustment with the accuracy of a pitch 0.1 .mu.m, the accuracy of approximately 6 minutes becomes necessary for the angle of rotation .theta. of the image, and a great burden is required for assembly adjustment.
Further, for the above-mentioned inter-spot spacing d to be created, an angle of view of approximately 30 minutes is necessary for the objective lens 25. To obtain a high resolution during information reproduction, a lens having NA as high as the order of 0.55 is used as the objective lens 25 and therefore, where said angle of view is as great as 30 minutes, it is difficult in lens designing to require good imaging of all spots, and to secure good imaging, the number of light beams passed through one and the same objective lens is limited to about two.
It is also possible to construct a plural-beam optical head by the use of an LD and beam dividing means utilizing a diffraction grating. As is well known, beam division by a diffraction grating is effected by an angle of diffraction conforming to the order number of diffraction and the angle of diffraction can be selected by the pitch of the diffraction grating and therefore, by making the angle of diffraction, i.e., the angle of division, small, the disadvantage as noted above is eliminated. This method, however, suffers from the following problems. Firstly, the diffraction grating greatly differs in diffraction intensity depending on each order number and is not suitable for the utilization of a plurality of beams equal in quantity of light to one another. Likewise, there is a loss of quantity of light by diffracted lights of the order number than the order number utilized. Secondly, the diffraction grating has its angle of diffraction fluctuated by the wavelength of light used and therefore, when the oscillation wavelength of LD becomes irregular or the fluctuation of the wavelength is caused by the temperature characteristic during operation or the like, the beam division angle is fluctuated.
Further, the division of a light beam can be accomplished by the use of a Wallaston prism. FIG. 2 of the accompanying drawings is an illustration of that method. A Wallaston prism 31 comprises triangular prisms 32 and 33 of two-axis crystal such as rock crystal cemented together so that the directions of their crystal main axes may be orthogonal to each other. The reference numerals 34 and 35 designate the directions of the crystal main axes of the triangular prism 32 and 33, respectively. The reference numeral 36 denotes the direction of polarization of the incident LD light, and this direction of polarization forms an angle .phi. with respect to the direction of the crystal main axis on the entrance surface of the prism. The incident light travels through the triangular prism 32 as ordinary light and extraordinary light whose directions of polarization are orthogonal to each other, and arrives at the boundary surface 37 between the prisms. Since the crystal main axis of the prism 33 is orthogonal to that of the prism 32, the relation between the ordinary light and the extraordinary light is inverted with respect to each polarized component before and after the incident light passes through the boundary surface 37 between the prisms, that is, the polarized component which has sensed an ordinary light refractive index n.sub.o in the prism 32 senses an extraordinary light refractive index n.sub.e in the prism 33, and the polarized component which has sensed the extraordinary light refractive index n.sub.e in the prism 32 senses the ordinary light refractive index n.sub.o in the prism 33 and thus, the two polarized components create refraction at different angles of refraction. Thereby the light beam is divided into two. Where the wavelength of the incident light is 780 nm and rock crystal (n.sub.o =1.53859 and n.sub.e =1.547949) is used as the prism material, the angle of division is about 1 degree. Also, the ratio of divided quantity of light is expressed as sin.sup.2 .phi.: cos.sup.2 .phi. by the angle .phi. of the direction of polarization of the incident light. Thereby, the divided quantity of light can be determined substantially freely.
However, the Wallaston prism is for the division into two beams, and for the division into more than two beams, a plurality of Wallaston prisms must be combined together, and this leads to complication of the construction of the optical head. Also, since the angle of division is about 1 degree, the angle formed between two beams is of the order of 30 minutes even after beam shaping means having a shaping ratio of the order of 2 is used, and as in the case of the aforedescribed LD array, a great burden is required for the assembly adjustment of the image rotating prism, and to secure good imaging, the number of light beams passed through the one and the same objective lens is limited to the order of two.
On the other hand, the division of a light beam can be accomplished by using, for example, a right triangular prism formed of single-axis crystal and by making the inclined surface of this prism into a total reflection surface and making the other two surfaces into an entrance surface and an exit surface, respectively, and making the direction of the crystal main axis of this prism coincident with the direction of the light beam reflected by the relation surface. Again in this method, however, use is made of only the maximum refractive index and the minimum refractive index obtained from a refractive index elliptical body of single-axis crystal and therefore, the number of beams provided by division is two, and the division into three or more beams is not effected and the angle of division is as great as the order of 1 degree.